However, on the iPhone 5/5c/5s (and any other LTE device) you can use a SIM card from the US. It has been shown to work, although, not without any issues. Your mileage may vary!

Voice too or just data? SInce voice is still on the CDMA parts of the phone, doesn't Verizon still require the device be OEMed for Verizon? Can one take an AT&T 5/5C/5S and put their Verizon SIM in it... and put the device's MEID on their account via MyVerizon and have a fully working device?

Voice and data, although data will be on its faux "4G" network (HSPA+)

As for an AT&T iPhone working on Verizon, no. AT&T Model A1428 can't be used for VZW, whereas Verizons Model A1429 can be used for just about any carrier, but it will lack support for certain features, in AT&Ts case, it's LTE network.